28 Apr

Thermostat wars – what if they’re not all in your head?

So we’re coming into the office air conditioning season after a warm dry spring. And that means it’s time for the annual crop of headlines about thermostat wars. Like this one, and this one and this one. Notice something similar with these?

That’s right – all of the ‘discussion’ about office temperature seems to be about the difference between people. Specifically, that men generally like it colder than women. Oh, and for some strange reason the ladies in marketing always feature as needing it hotter than anyone else.

Folks, I smell a giant, slightly sweaty, rat here. Of course, there are differences in how people perceive temperature. However, if you’ve had the pleasure of ploughing through the ASHRAE and CIBSE literature on temperature and comfort in offices, then you know that an immense amount of research effort goes into identifying a range of temperatures that works for the largest number of people.

So when you get surveys that suggest 60% of people are unhappy with the temperature in their office – cooling in summer or heating in winter – my conclusion has to be that its much more likely the problem is the HVAC.

Too hot and too cold. In the same office?

People disagreeing about office temperature and fighting over the thermostat makes good headlines. Truth is, they may both be right. Its completely normal for office temperatures to vary across the room. So Karen sitting under an air conditioning vent may well be freezing while Mick starts to sweat in the sun at the other side of the room.

And while humans aren’t very good at estimating absolute temperature, we have quite high levels of sensitivity to changes in temperature – anything over 1 degree in 15 minutes is generally detectable. So even quite small variations throughout the day tend to get noticed, particularly if they coincide with peaks and troughs in metabolism.

Why does my office temperature vary so much?

Temperature – and how it affects comfort – is a more complicated subject than it might first appear. If we break it down to its component parts it begins to be obvious why variations happen in buildings.

The basic physics of how heat moves around is probably familiar. Heat can be transferred from one thing to another by radiation (heat energy being transferred directly – like in sunlight), convection (heat being carried around in air or other fluid) or by conduction (transfer along a physical object like a metal spoon in coffee). In an office, radiant heat and convection are normally the two most significant mechanisms for transferring heat around, and these create the first set of challenges.

Because heat will move around a bit differently, depending on the mechanism. Unless you’re sitting in an office made of warm water convection is going to rely on warm air circulating around your office. On the other hand, any ‘body’ – a window, a wall, a desk, a laptop – that you can see will be radiating heat directly at you. If they are at a similar temperature to you this effect will be small, but once the mean radiant temperature gets more than 5 degrees out of whack to air temperature, you are likely to feel uncomfortable.

Since the biggest ‘bodies’ in any office are generally windows and walls, and since they are generally exposed to external temperature, being close to a hot window or a poorly insulated wall makes you very vulnerable to fluctuations in their temperature. And you might be freezing while Frank near the kitchen is toasty.

But wait, there’s more! Why your office’s temperature varies over time.

Do you like graphs? Of course you do. Here’s a fun one:

This is a graph of the temperature measured at multiple points across an open plan office for 19 days. Spot the two weekend periods (here’s a clue – look at the minimum temperatures). Why are they significantly cooler? It’s not just because there is no heating on. In fact the main difference is the lack of people. Occupation adds heat. A lot of heat. Each person gives out about the same about of heat as a 100 Watt lightbulb, and that’s before you add in the lighting and equipment that they need.

On top of that, there are long term trends that change how hot offices get, from cooler technology – computer monitors that use LED screens – to hot desking. So over the course of the day and over time you can expect to see the heat load (how much heat is generated) change a lot.

Surely my office heating and ventilation is designed to deal with all this?

Indeed it is. And in future blog posts we’ll have a look at some of the work on how zoning, building management systems and natural ventilation is helping.

However, as you can see, how comfortable you are during the working day is actually the sum of some complicated factors, and that’s before we get into activity levels, clothing, humidity, air speed and all the other issues that have significant impact on the working environment. A good building should compensate for many of these, but there are multiple reasons why they change over time, so regular reviews of whether the building’s heating and cooling is doing its job is always a good policy.

Not just telling the lady in marketing to put on another coat.

This is just the first in what we hope will be a series of posts about HVAC and comfortable buildings. Tell us what problems you’re having and we’ll make sure we cover them.